This invention relates to organic electrolyte batteries employing an active material of negative electrode made of metallic lithium, lithium alloy, or intercalation compound formed between lithium and carbon or lithium and metal oxide and an organic electrolyte, enabling long period use and/or storage under a high-temperature and high-humidity environment.
Whereas organic electrolyte or solid polymer electrolyte has been used as the non-aqueous electrolyte for battery, the organic electrolyte has been prepared by dissolving lithium salt as a solute into an aprotic organic solvent because of its high stability even in the very active metallic lithium and composition having a low melting point.
Because of its excellent low temperature characteristics, a wide operating temperature range, and the superior long-period storage characteristics of the organic electrolyte batteries, it have been used extensively not only as the main power sources for various applications such as consumer-use electronic watch, electronic note-book, auto-focus camera, and etc., but as the memory back-up power-supplies incorporated in various electronic circuits which is expanding very rapidly in recent.
As for these backup power-supplies, not only the lithium-graphite fluoride, Li/(CF).sub.n and lithium-manganese dioxide, Li/MnO.sub.2 system primary batteries, but more of the lithium secondary batteries are used recently.
A typical longitudinal cross-sectional view of Li/(CF).sub.n system coin-type organic electrolyte lithium cell is shown in FIG. 1 wherein the metallic lithium acting as negative electrode 2 is compressed in one body to the inner surface of cover case 1 made of stainless steel acting as a negative terminal, gasket 6 is mated on the flange of cover case 1, positive electrode 3 molded mainly of (CF).sub.n active material is pressed on a current collector made of titanium lath connected on the inner bottom surface of case 5 made of stainless-steel (made of SUS 304; containing Cr 18%, Ni 8%) acting as a positive terminal, and negative electrode 2 and positive electrode 3 are separated by separator 4.
The organic electrolyte is impregnated and held within a pore formed between the positive electrode 3 and the separator 4. The flange of metal case 5 is curled and pressed on the gasket 6 placed between the cover 1 and the case 5 to seal the cell.
Conventionally, sealant layer 8 made mainly of blown-asphalt is disposed in advance et least on the surface of gasket 6 contacted with cover case 1 in order to improve the sealing performance. In a conventional Li/(CF).sub.n system organic electrolyte lithium cell, a separator made of polypropylene (PP) non-woven cloth, a gasket made mainly of polypropylene (PP) resin, and an organic electrolyte prepared by dissolving a solute of lithium fluoborate (LiBF.sub.4) into a high-boiling point solvent, .gamma.-butylolactone (BL), or into a mixed solvent consisted of BL end a low boiling point solvent 1,2-dimethoxyethane (DME) mixed at a concentration of 1.0 mol/l has been used.
The thermal decomposition temperature of graphite fluoborate employed as a positive active material in Li/(CF).sub.n system cell employing organic electrolyte is within a temperature range from 320.degree. to 420.degree. C. producing no melting deformation up to the melting point of metallic lithium which is 180.54.degree. C. when it is used as the negative electrode.
Since the thermal decomposition temperature of lithium fluoride (LiF) which is a discharge product is 848.degree. C., the Li/(CF).sub.n system cell can be primarily thermally stable cell. Thus, the long-period storage and/or use of conventional Li/(CF).sub.n system cell for more than 10 years in a temperature range of -40.degree..about.+60.degree. C. had been possible.
The prevention of external leakage of internal cell-elements such as the electrolyte and the liquid- and gas-tightnesses preventing the intrusion of external air and moisture into the cell can be realized generally by applying a sealing such as a gasket made of synthetic resin having a high resistance against the electrolyte and high electrical insulation characteristics between the metal case acting as a positive terminal and the metal case acting as a negative terminal regardless the types of cell including the coin-type, button type, wafer-type, and the cylindrical type.
With the recent rapid advancement of electronics technologies, the field demanding memory-backup cells have been expanding very rapidly. Accordingly, the cell has to withstand against more rigorous conditions such as the higher-temperature and higher humidity possible within automobile engine compartment or outdoor environments in which industrial equipments are installed and operated.
Moreover, no deterioration of the cell characteristics is permissible even when the cell is mounted on a printed circuit board together with other electronic components being exposed to a rigorous high temperature condition of reflow soldering.
For the organic electrolyte other than that for the Li/(CF).sub.n system, a solvent obtained by mixing propylene carbonate (PC) having a high-boiling point with 1,2-dimethoxyethane (DME) having a low boiling point is used extensively. The purpose of the addition of low boiling point solvent is to lower the viscosity of electrolyte and to improve the high-rate discharge characteristics including the pulse discharge characteristics.
However, when the coin-type or wafer-type cell having a larger cell cross-section compared with the thickness of cell container is placed in a high-temperature environment, the low-boiling point solvent in the electrolyte would be gasified increasing the internal pressure of cell and swelling the cell container. In an extreme case, this may produce a gap in the seal causing a leak of the vapor of the low boiling point solvent.
At this condition, a number of problems such as the deteriorations of cell characteristics including the change of electrolyte composition, leak of organic electrolyte itself, intrusion of air from outside, or an increase of internal resistance caused by en oxide layer formed on the lithium negative electrode due to the moisture contained in air, are possible.
In order to solve these problems, the cell having a high heat-resistance has to be developed. In order to accomplish this, extensive efforts improving the heat-resistance of organic cell-materials such as the gasket, sealant, separator, etc., including the composition of electrolyte had been made. For example, the U.S. Pat. No. 5,246,795 and Japanese Patent Publication Hei 5-58232 show proposals resulting from these efforts, but the practical effects of these can never be satisfactory.
Furthermore, a cell of hermetically sealed construction employing a cover on which the positive terminal is glass-to-metal sealed is welded on the upper flange of negative polarity case by laser had been developed. Although the reliability of this structure is excellent, the cost of components is almost forbidding for producing the general purpose cells.
Since the melting point of PP resin used conventionally for making the separator or gasket in the Li/(CF).sub.n system organic electrolyte cell is relatively high, this has been used as a general purpose resin of high heat-resistance. Whereas the continuous service of mold of PP resin is possible at conditions of and a relatively low loading stress and is possible even at a temperature as high as 150.degree. C. if no load is applied. However, while deformation of the gasket due to cold flow is possible even at a temperature less than 100.degree. C. if gasket is subjected to a high loading stress. Thus, the above mentioned problem could be possible when the cell is subjected to a long storage period and/or use condition at a higher temperature exceeding 60.degree. C.